This invention relates to a method for improving the adhesion obtained between an adhesive of the class of polymerizable liquid ethylenically unsaturated compounds containing a single terminal &gt;C=CH.sub.2 group, (referred to as the adhesive hereinafter), and a solid polyester or thermoplastic substrate and to a solution of a partial urethane adduct utilized to treat the polyester or thermoplastic substrate prior to application of the adhesive to effect the desired improvement in adhesion.
Molded or laminated polyester compositions, especially fiber reinforced polyester compositions and thermoplastic compositions (referred to as "FRP" and "plastic " compositions hereinafter), are widely utilized today to form assemblies which make up automobile body panels, truck cabs, boats and the like. The preferred method of joining molded FRP or plastics to form the desired assemblies is with a structural adhesive. It has been determined that such adhesives act to evenly distribute throughout the area of attachment stresses encountered when the assemblies are subjected to normal vibration, shocks and forces encountered when in use as part of an operating vehicle, for example. In contrast, in assemblies formed with conventional techniques such as rivets and screws, stresses encountered in use concentrate at the points of attachment, causing the holes required for the rivets and screws to crack or widen. Since the FRP or plastic is not as strong as the metal rivets or screws, holes in the FRP or plastic wear until the metal fasteners can fail by being pulled out of the seam holes.
Joining molded FRP or plastics into desired assemblies with structural adhesives is not without problems. The normally smooth surface of inert, molded FRP or plastic substrates does not foster the formation of a secure and enduring bond between the structural adhesive and the FRP or plastic substrate. Various techniques have been employed to promote adhesion between adhesives and FRP or plastic substrates.
A common practice, especially during early use of FRP and plastic assemblies, involved abrading of the bond area of the FRP or plastic substrate by grit or sandblasting to provide a surface more amenable to adhesive assembly. This technique has severe limitations and is falling into disuse. Surface abrasion by grit or sandblasting in the bond area weakens the FRP or plastic compositions in the precise areas where maximum strength is needed and renders the bond area susceptible to absorption of paint solvents which can weaken and attack the FRP or plastic. Absorbed paint solvents are especially deleterious since they can re-emerge against the structural adhesive interface causing deterioration of the adhesive bond during high temperature bake cycles used to cure the paint and primer applied to a finished assembly. Of course, abrading the bond area is costly since it involves an additional processing step and requires skilled labor to prepare the seams without destroying the cosmetic surface contour of the soon to be painted exterior automobile or truck body panels. Also, the fiberglass particles created during blasting of FRP compositions are a severe health hazard, requiring protective equipment for workers and expensive exhaust and removal equipment.
Also, the current widespread use of FRP compositions made with "low profile" polyester resins have made the use of surface abrasion techniques impossible due to unacceptable strength loss of the FRP. Low profile polyester resins are polyester resins filled or diluted with thermoplastic polymers such as polyvinyl chloride. The thermoplastic diluent prevents the polyester resin from shrinking away from the surface of the reinforcing glass fibers which are incorporated in the resin to give it strength. The reduced shrinkage gives the polyester resin a "low profile" or very smooth regular surface as contrasted to the "high" or "standard" profile of usual FRP resins which have irregularly contoured surfaces as the result of exposed reinforcing glass fibers in the resin. Low profile FRP compositions are especially desirable in forming automotive assemblies since they yield better painted surfaces with little of the previously required sanding and preparation prior to painting. However, the low profile FRP compositions are somewhat lower in strength than standard FRP compositions due to the polyvinyl chloride diluent. Thus, any strength loss due to abrasion of the low profile FRP is unacceptable.
With the recognition of the serious problems created with sandblasting, and the like, attempts have been made to develop alternative techniques for preparing FRP and plastic substrates for surface bending with adhesives. Various treating agents or primers such as the tertiary amines described in U.S. Pat. No. 3,647,513 have been applied to FRP surfaces prior to application of certain urethane adhesives. However, it was found that the amines and solvents do not adequately soften the FRp substrates in the area of attachment and cause severe weakening of plastic substrates. Further, amines frequently inhibit curing of adhesives such as those described in this specification.
A solution of a polyisocyanate trichloroethylene has also been utilized as a surface primer in joining FRP compositions. However, FRP compositions treated with the polyisocyanate solution had to be adhesively joined within the narrow period between the time the polyisocyanate primer initially hardened, usually after about 2 days of open air moisture cure, and the time several days after application of the solution when the polyisocyanate primer coating becomes so hardened and inert that adhesive bonds cannot be formed with it.
Thus, it can be seen that the art is still searching for suitable agents for treating FRP and thermoplastic substrates to make them amenable to adhesive bonding. An ideal treating agent or primer for FRP or thermoplastic compositions should meet several criteria. The primer should eliminate any need for abrading or other surface cleaning procedures prior to application of the adhesive. Secondly, the primer must be in such a form that it can be conveniently and economically applied to the polyester or thermoplastic substrate and not require any special post application treatment such as baking, for example, to effect a bond with the polyester or thermoplastic substrate. In some instances it is necessary that the primer be ready for coating with an adhesive within a short time after its application to the polyester or thermoplastic components. Thus, in such cases, the primer should be one to which adhesives can be applied in a dried, solvent free, but uncured state. Lastly, the primer must have a sufficient pot life so that it can be prepared and utilized over a period of time sufficient to accommodate at least one work shift or preferably at least 12 hours.
It is the object of this invention to provide a method for improving the adhesion of adhesives to solid FRP or thermoplastic compositions which does not require that the surface of the FRP or thermoplastic be abraded or roughened prior to application of a structural adhesive. It is a further object of this invention to provide a composition useful to prime the surface of FRP or thermoplastic compositions to improve the adhesion obtained with the application of adhesives to the FRP or thermoplastic composition. Still another object of this invention is to produce a solution of a partial urethane adduct useful as a primer for solid FRP or thermoplastic compositions which satisfies the general criteria noted above.